Proc. Natl Acad. Sci. USA 112, 7444–7448 (2015)

When you want to understand a process, sometimes there's no better way than observing it directly. Of course, if it's the laser-induced melting of a nanoparticle you're interested in, simply looking is far from straightforward. But Jesse Clark and colleagues have now managed to do exactly that — imaging the reversible melting of a single gold nanocrystal on picosecond timescales using an X-ray free electron laser.

Theory, molecular dynamics simulations and experiments on nanoparticle ensembles have together succeeded in providing us with a picture of what nanoparticle melting might look like. The common wisdom on the strength of these findings holds that isolated regions on the surface of the nanoparticle melt first. Measurements from previous pump–probe experiments suggest that this process might initiate at melting temperatures as low as 70% of that of the bulk.

By probing a single nanoparticle, the team was able to obtain images of the melting transition with unprecedented temporal resolution, and confirm that the partial melting process is indeed non-homogeneous — and also fully reversible.